As a conventional stratified scavenging two-cycle engine of this kind, a stratified scavenging two-cycle engine that includes a scavenging flow passage for connection between a cylinder chamber and a crank chamber and an air flow passage connected to the scavenging flow passage and that is structured in such a manner that pressure reduction in the crank chamber, with upward movement of a piston, permits a fuel mixture to be drawn into the crank chamber and permits air to be drawn into the crank chamber, through the scavenging flow passage from the air flow passage, is known. In the stratified scavenging two-cycle engine structured as described above, there is an advantage that combustion gas can be pushed out by air from the scavenging flow passage, thus making exhaust gas cleaner by greatly reducing an introduction of a fuel mixture during combustion gas expulsion.
In the aforesaid stratified scavenging two-cycle engine, however, there is a disadvantage that the fuel mixture is rarefied by air, whereby an air-fuel ratio (weight of air/weight of fuel) having a substantial ratio of air to fuel becomes thinner (increases), thus deteriorating acceleration performance. As a measure to improve acceleration performance, it is required that the air-fuel ratio is thickened (decreases) by increasing the supply amount of fuel also at a time of stationary engine speed in accordance with acceleration performance to draw an enriched fuel mixture into the crank chamber. In that case, however, an exhaust gas quality at the time of a stationary engine speed (i.e., other than a time of acceleration) deteriorates.